Features of the polyamine/epoxy resin systems known from the prior art include, for example, excellent metal adhesion, very good chemicals resistance and outstanding corrosion protection properties. In the case of solvent-borne formulations and powder coating systems it is possible using epoxy resins with high molar masses and/or polyaminoamides, based for example on dimer fatty acid hardeners, to obtain crosslinked films of high flexibility. Coatings based on solvent-free liquid resins and solvent-free amine-type hardeners, owing to the low molar masses of the epoxy resins and the resulting high network density, are very brittle. In solvent-free formulations, therefore, use is made of, for example, tar substitutes such as coumarone resins for plasticization. Especially when relatively large amounts of hydrocarbon resins are used, however, coatings of this kind tend towards long-term embrittlement owing to migration of the non-functional constituents.
Effective and long-lasting elasticization of the epoxy resins can be achieved through combination with blocked polyurethane prepolymers.
DE-A 21 52 606 describes in this context the use of alkylphenol-blocked polyisocyanates, the reactive systems produced (e.g. coating materials or composite materials) being of relatively high viscosity.
An improvement in this respect can be obtained, according to the teaching of U.S. Pat. No. 6,060,574, by using hydrocarbon-resin-blocked polyurethane prepolymers, in which case reactive systems of lower viscosity are obtained.
Catalysts described as suitable for the blocking reaction in DE-A 21 52 606 and U.S. Pat. No. 6,060,574 are, very generally, the catalysts which are customary per se in polyurethane chemistry, including Sn(II) or Sn(IV) compounds or tertiary amines such as 1,4-diazabicyclo[2.2.2]octane (DABCO) or triethylamine.
However, the only two tertiary amines mentioned explicitly, DABCO and NEt3, have unwanted drawbacks. For instance, the deactivation of the catalyst after the blocking reaction, which is normally carried out with acid chlorides such as benzyl chloride, leads to unwanted precipitation and clouding phenomena in the product, so making it unusable. Owing to its high volatility (boiling point: 88–90° C.), in contrast, triethylamine is readily expelled from the reaction mixture during the blocking reaction (reaction temperatures typically up to 90–120° C.), and this leads, among other things, to a reaction rate which is not uniform, is difficult to control and in some cases is severely retarded.